The problem you'll run into with mixing them is the chickens will toss out and waste a lot of food looking for all the scratch grains. I keep the feeder filled with layer and just toss out a couple handfuls of scratch in the run each morning.
Listen to cafarmgirl... It'd be like mixing brussel sprouts with M&Ms; we're all going to dig through the brussel sprouts to get to the M&Ms. They would fill up (if chickens ever actually fill up...lol) on the scratch rather than eat the layer feed. Of course I'm referring to the scratch sold by most feed stores, which had very low protein levels (as opposed to layer feed's 16+%). Scratch mixed fresh can have as high a protein level as you want it to, depending on what you mix in. I would not advise mixing your scratch with your layer feed. What would be the purpose??? Scratch is generally used as a treat only.
When you add scratch to the feed you will deplete some of the nutrition of the layer feed including the protein.
You can look at it like this, if you feed a 16% protein layer and mix it 50/50 with scratch (8% protein) your feed is now 12% protein. A big drop